1. Field of the Invention
This invention relates to the precision replication production of relatively dense, dimensionally precise articles by chemical vapor deposition. The production method involves chemical vapor deposition of the article on a suitable substrate and the subsequent separation of the article having a replicated surface in its essentially final shape and finish from the substrate. Particularly advantageous embodiments of the invention involve the production of optical components or articles such as cones, cylinders, domes, aspheric surfaces and small f-number optics which have steep curvatures. Specific applications of such components include lenses and mirrors for cameras, imaging systems, astronomical telescopes, synchrotrons, conformal domes for missiles and torroidal windows for aircraft. The invention is particularly suitable for producing infrared optical articles of zinc sulfide.
2. Description of Related Art
Chemical vapor deposition (CVD) has been used to provide various materials either in free-standing bulk form or as a layered coating on a substrate. CVD methods of providing zinc sulfide layers and articles are described in commonly assigned U.S. Pat. Nos. 4,978,577 and 5,686,195, and in the references cited therein. Generally, the previous methods of producing bulk material by CVD processing have provided solid shapes, the surfaces of which are then machined to their final article form. Replication techniques for producing "near-net shape parts" which minimize machining are suggested and some results described in Goela et al., "CVD Replication for Optics Applications", SPIE Proc., 1047, pages 198-210 (1989). An improved replication process for producing silicon carbide articles described in U.S. Pat. No. 4,997,678 provides an in situ applied coating of carbon on a polished preshaped substrate prior to initiating the chemical vapor deposition of silicon carbide to form an article replicating a surface of the substrate.
While the techniques described in the prior art have produced near-net shape articles, there continues to be a need for a precision replication technique which will provide precisely shaped articles, such as optical components, without the need for final machining of the optical surfaces. The techniques described in U.S. Pat. No. 4,997,678 provide relatively good replicas, however the carbon rich film used therein is applied in situ at the beginning of each production deposit. Such does not provide an opportunity to measure or otherwise verify the dimensions of the film prior to commencing the production run, and allows uneven growth of the carbon rich film to result in uncontrolled deviations in the replicated article. Moreover, the carbon rich film tends to adhere to the surface of the replica when such is separated from the substrate. These characteristics tend to limit the ability of this technique to replicate the finish of the substrate.
Techniques which provide precision replication of, and facile release from, a substrate are particularly needed. For instance, infrared sensors used for navigation, guidance and targeting on aircraft and missiles require protection from the elements by transmissive windows or domes. Preferably such windows or domes are provided in a shape which minimizes aerodynamic drag while avoiding transmission irregularities. In a missile, the preferred location is in the nose. A spherical dome at this location produces considerable drag which can be significantly reduced by the use of an extended generally conical shaped dome. The fabrication of such a dome, however, is made difficult, or even impossible, when machining of the interior surface of the dome is required since the functionality of the required apparatus decreases as the diameter of the cone decreases and its length is extended. The precision replication of such interior surface of a dome on a reusable mandrel, such that machining or polishing of such surface is not required, would provide significant fabrication advantages. Certain CVD produced bulk materials, such as zinc sulfide and zinc selenide, however, due to their thermal expansion characteristics and those of their preferred mandrel materials, have not previously been produced on the exterior surfaces of a curved male mandrel. Instead, when such materials have been used to produce curved products, they have been fabricated by deposits formed on the interior surfaces of curved female mandrels, whereby the interior surfaces of such curved articles require considerable machining to provide their required final figure and finish.